Modern flash memory devices may store information as charge in cells, where each cell may be made of a floating gate transistor or an NROM transistor. In single-level cell (SLC) flash memory devices, each cell may store only one bit of information. Multi-level cell (MLC) flash memory devices can store more than one bit per cell by choosing between more than one pair of levels of electrical charge to apply to the floating gates of their cells. The applied charge (also known as charge level) may be measured by a detector. The detector may compare the voltage of the transistor gate (also known as charge level and denoted VT) to a decision threshold voltage (also known as charge level boundary point and denoted VD). The amount of charge may be used to determine the programmed level (logical value) of the cell. Due to inaccuracies during the programming procedure and charge loss due to time and temperature (also known as retention), the measured levels may suffer from a random distortion.
In multi-level cell flash memory devices, each cell can store more than one bit per cell. The program level of each cell is determined by an n-tuple of bits (e.g., n=1, 2, 3). The cell might not be directly programmed to one of its possible program levels in a single step. Program levels may be written in stages, where in each stage only a single bit is programmed.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.